Impregnated fibrous wallboard and method of making



Patented Feb. 24, 1953 angers IMPREGNATED FIBROUS WALLBOARD AND METHODOF MAKING Walter P. Ericks, Lockport, N. 17,, assignor to The UpsonCompany, Lockport, N. IL, a corporation of New York No Drawing.Application July 7, 1949, Serial No. 103,528

19 Claims.

This invention relates to dimensionally stabilized materials ofcellulose fibers, particularly cellulose structural materials, and tomethods for stabilizing such materials against dimensional change causedby change in the humidity of the environment surrounding such cellulosematerials. More particularly, the invention relates to the stabilizationof structural cellulose fiber boards as well as wood and paper andfabrics made of cotton, linen and other cellulose mate? rials to renderthem more resistant to dimensional changes resulting from variations inthe ambient humidity and to improve the strength of such products.

It is well known that materials made up entirely or predominantly of.cellulose fibers expand and contract with variations in humidity in theambient atmosphere, such materials sufiering an increase in theirdimension upon absorption or moisture from the atmosphere and acontraction when moisture is given up to the atmosphere upon a decreasein the humidity thereof. It is also well known that in articles whereinfibers are directionally oriented, such expansion and contractionusually occurs to the greatest extent in a direction perpendicular tothe predominant direction of the fibers. The present invention is,therefore, adapted particularly in preventing or minimizing thedimensional change which occurs across the fibers with change inhumidity in cellulose materials, al thcugh it also reduces dimensionalchange in the direction of the fibers with humidity change.

Various expedients have been heretofore employed for the purpose ofdimensionally stabilizing materials made up predominantly of collu losefibers as, for instance, plywood, wood boards, pulp products andcombinations thereof, and solid paper boards. A degree of dimensionalstabilization is obtained in the manufacture of plied or laminatedarticles by arranging the laminations with their fiber directionsdisposed angularly to one another rather than parallel. Althoughimprovement in dimensional stabilization is obtained, the operation islaborious since it requires cutting and proper selection and assemblageof the plies.

It has also been suggested to densify the products under heavy pressureand to thereby set he cellulose fibers. Very expensive presses andextensive auxiliary equipment is required for thisoperaticn, and theproduct lacks low density and some of the flexibility desired for manyuses of structural cellulose fiber board.

In my copending applicatiqn Serial No.

627,966, filed November 10, 1945, and now abandoned, of which this is acontinuation-in-part, I have disclosed that certain organic compoundshaving at least two hydroxyl groups in their molecules, particularlypartial esters of polycarboxylic acids and polyhydric alcohols having atleast one hydroxyl group in the residue derived from the polyhydricalcohol and at least one carboxyl group in the residue derived from thecarboxylic acid, stabilize structures made up predominantly of cellulosefibers against expansion and contraction due to variation in atmospherichumidity. I have also disclosed in my copending application Serial No.103,526 filed July '7, 1949, that other compounds having both hydroxyland carboxyl groups, such as hydroxy carboxylic acids, are alsoeffective stabilizers against such expansion and contraction. Also in mycopending application Serial No. 103,527, filed July 7, 1949, I havedisclosed that polyhydric alcohols have a similar stabilizing effect.

In accordance with the present invention, I have found that materialsmade up predominantly of cellulose fibers may be wholly or partiallystabilized against dimensional change by introducing into such cellulosematerials certain specified chemical compounds which also appear to havea particular affinity for the cellulose fibers. Compounds which producedimensional stabilization are organic in nature and have at least twohydroxyl groups, at least one of which is a part of a carboxyl group,and possess certain other characteristics with respect to volatility.The series of compounds possessing dimensional stabilizingcharacteristics are the polycarboxylic acids.

In this case, as well as in that dealing with the partial esters and thehydroxy carboxylic acids mentioned above, at least one of the hydroxylgroups forms a part of a carboxyl group. The polycarboxylic acids areusually either soluble in water or in low molecular weight aliphaticalcohols or ketones or mixtures of these solvents. When in solventsolution, they rapidly penetrate into the fibrous structures, betweenthe fibers and into the fiber cells, and in fact many of them rapidlypenetrate such fibrous structures even in the absence of a solvent. Inmost cases, however, greater dimensional stability is obtained when thepolycarboxylic acids are employed in solution in a solvent forimpregnating the fibrous structures. Further properties andcharacteristics of the stabilizing chemicals will be more fullydescribed hereinafter.

For purposes of illustration only, the invenvention will be described indetail in its application to the production of dimensional stability inlaminated structural cellulose fiber boards. Such products are bestexemplified upon the market by the structural building panels sold underthe name Upson Board. These cellulose fiber boards are generally manufactured from so-called fiber boards, that is, a fiber sheet with acaliper greater than about 0.030 inch. These fiber boards are assembledand bonded to one another to produce a laminated or plied articlehaving, for instance, from two to about seven plies. The resultinglaminated structural cellulose fiber board occurs in standard sizedpanels of from inch to inch or more in thickness, and of specifiedlengths and widths. The original cellulose board is manufactured fromany conventional type of cellulose pulp stock as, for instance, groundwood fiber, chemical wood fiber, rag fiber and other conventional pulpfibers and mixtures thereof. The initial cellulose board whichconstitutes the individual ply may be made either upon a conventionalcylinder machine, as is generally the case, or may be made upon aFourdrinier machine. It will be understood, however, that the inventionis of general application to structural cellulose materials as, forinstance, fiber insulation board, sound absorbing board, table topboard, structural board for the interior of an airplane, and the like.

The compounds employed to efiect stabilization in the structuralcellulose fiber board against dimensional change induced by change inhumidity may be introduced into the fibers from which the board is made,into the individual plies of the ultimate laminated structure or intothe final laminated assembly itself. The choice of the place ofintroduction of the stabilizing compound and the manner in which it isto be introduced will be dictated by the type of fiber available and thetype of structural panel to be produced.

Thus, when operating a closed board machine system wherein all water isrecycled, the impregnating compound may be added to the beater or to thestock prior to paper formation, as for instance in the head chest,assuming that a stabilizing compound has been chosen which is notreadily subject to hydrolytic change at the temperature and pH of thepulp suspension. Or, the impregnating compound may be added at any otherpoint in the wet end of the machine.

Where the individual cellulose structural board is already formed, thestabilizing compounds may be introduced into the board by immersing theboard in the compound or a solution thereof or by impregnating the boardwith a spray containing the treating compound or by applying it withpatting rolls, all conventional methods of impregnation. Where alaminated board has already been formed by bonding a plurality ofindividual boards together, the resultant laminated article may beimmersed in the stabilizing compounds or a solution thereof and theimpregnated board subsequently dried. The impregnation under suchcircumstances will generally be desirably performed by subjecting theboard to vacuum, at which time occluded gases and volatile materials areremoved from the board, then permitting the impregnating solution toflow into the evacuated chamber containing the board generally placedtherein in an upright position and spaced apart, whereby the 4 boardsare enveloped in the solution which is, in effect, forced into theboards. This penetration of the solution may then be increased byrelieving the vacuum and, if desired, raising the pressure above that ofthe atmosphere to enhance the speed and depth of penetration.

It is therefor an object of the present invention to provide a simpleand inexpensive impregnating method for dimensionally stabilizing andstrengthening structures made up substantially of cellulose and toproduce dimensionally stabilized cellulose products.

In broad aspect, therefore, the invention comprehends the incorporationinto structural cellulose fibrous materials of a polycarboxylic acid ormixture of such acids whereby the usual expansion and contraction ofsuch cellulose materials is considerably minimized by reason of changein humidity conditions in the atmosphere surrounding such materials.This stabilizing effect is dependent upon the quantity of the stabilizing compound incorporated in the fibrous cellulose structuralelement. Effective dimensional stabilization has been accomplished byincorporating in the fibrous material from about 2 to 50% of thestabilizing compound based upon the weight of oven dried fiber. Theexact quantity to be incorporated into the cellulose material Will bedictated by the type of material, the type of polycarboxylic acidemployed as a stabilizing compound, and the amount of the usualexpansion or contraction which it is desired to remove. Thus, undercertain conditions of use, the removal of so little as 12 or 15% of thenormal expansion or contraction of a cellulose structural material maybe suitable, while in other conditions of use, it may be desired toremove 50, 60 or 70% or more of the normal expansion or contractionencountered with a particular change in humidity conditions in thesurrounding atmosphere. v

The stabilizing compound may be incorporated into the cellulose fibers,whether the same be in fibrous form, unfibrated or felted fibrous form,by the employment of aqueous solutions, solutions in hydrophilicsolvents, or mixtures thereof with water or in some instances may beincorporated without the employment of a solvent. However, the usualmode of incorporation will be to employ as an impregnating solution anaqueous or hydrophilic solution of the stabilizing compound.

The cellulose structural materials may be treated with the impregnatedstabilizing material or solution thereof at substantially any desiredtemperature, although the usual impregnating temperatures will rangebetween 20 C. and 50 C. However, temperatures as high as C. mayfrequently be employed.

While the actual mechanism of the stabilizing action of the presentinvention for cellulose fibers is not fully understood, it is believedthat their penetrating power and their fixation on and in the cellulosefibers is due to the particular molecular structure, that is to say, thepresence of hydroxyl groups in both the cellulose and the stabilizingmaterial.

After their incorporation in the cellulose material to be dimensionallystabilized, the stabiliz ing compounds show considerable resistance toremoval by water and solvents, and it is believed, therefore, thatprobably there is some loose form of physico-chemical combinationbetween the cellulose molecule andthe stabilizing chemical. Thisresistance to removal of the stabilizer by water and solvents is quitemarked, particularly if the impregnated cellulose prcductsarc. heated toelevated temperatures as, .for instance, between 100 C. and 200 C. It isfurther believed that the fixation of the stabilizing materials in andon the cellulose fiber may be due to the ability of the molecules of thestabilizing materials to react with each other, as well as wlth'thecellulose, whereby polymerization takes place with the formation of longchain molecules of high molecular weight. The presence of carboxylgroups in the polycarboxylic acid suggests that, on heating, thehydroxyl groups of the polycarboxyl acid reacts with the hydroxyl groupsof the cellulose to modify the chemical structure of the latter. It isbelieved that such modification of the cellulose results in increaseddimensional stability of the cellulose structural material and alsoincreases its strength and water resistance.

The fixation of the stabilizing compounds in and on the cellulose fiberscan be enhanced by employing them in combination with thermoset" tingresins, which in their partially reacted state are soluble in thevolatile, hydrophilic solvents for the stabilizers such aswater, lowmolecular weight alcohols and ketones or mixtures thereof. Thethermosetting resins, after setting, are believed to cover and protectthe stabilizing materials in and on the cellulose fibers from attack bysolvents. In this connection, it is further believed that thestabilizing materials penetrate farther into the cellulose fibers thanthe thermosetting resins, thus producing a protective coat ing ofthermosetting resins.

The incorporation of thermosetting resins into the cellulose structuremodifies to some extent the effect of the polycarboxylic acid in such away that the hardness and water resistance of the resulting cellulosefiber structures impregnated by the stabilizers are increased. Therequirement of the properties determined by the ultimate use of theresulting article will guide the selection of the stabilizing materialand its use either separately or jointly with a thermosetting resin.

Suitable thermosetting resins which may be employed in combination withthe stabilizing material of the present invention include phenolformaldehyde, urea formaldehyde, and melamine formaldehyde, which aresoluble in the volatile, hydrophilic solvents employed. Any otherthermosetting resins which in their partially reacted state have theproperty of being soluble in such solvents may also be employed. Theresins become insoluble and infusible upon advancement and preventattack by water or solvents upon the stabilizing materials and reactionproducts there of deposited in and on the cellulose fibers. The amountof thermosetting resin may be varied within a considerable range, forexample, between 5% and 50% of thermosetting resin in the finalcellulose fiber product based on the dry weight of fiber.

The advantages gained in impregnating articles made up of cellulosefibers with polycarboxylic acids alone and in combination withthermosetting resins are shown by the following examples which are to betaken in an illustrative rather than a limiting sense. In securing thedata for such examples the procedure outlined below was followed.

A board prepared on a cylinder paper machine from used news fiber wascut into strips measuring 0.051 x 2 x 12", extending in its largestdimension perpendicular to the predominating direction of fibers in theboard. The strips were aceaecs immersed intov the impre natin olutio k tat 50 C. and they were allowed to remain therein until the board waswetted to its center.

The time required for complete impregnation was within a range of 1 to 8minutes, and on the average was about 4 minutes.

The dry strip was weighed beforeimpregnation and, after the immersion inthe impregnant, dried by heating at 125 C. for 30 minutes. From thedifference in weight, the quantity of active ingredients depositedwithin the board, in and on the fibers, was determined. The strips wereaccurately measured dry and then conditioned in a humidifying chamber,kept. at relative humiclity and 38 C. for about 48 hours, at the end ofwhich perlodthe strips had absorbed a maximum of moisture and usuallyshowed no further increase in expansion. The total expansion of each ofthe unimpregnated control strip and of the impregnated and dimensionallystabilized strip was thus determined. The difierence in the amount ofexpansion between the control strip and the stabilized strip representsthe amount of normal expansion removed by means of the dimensionalstabilization treatment; the difierence in expansion between the twostrips divided by the total expansion of the control strip times is thepercent of normal expansion removed.

The fiexural strength of the strip was determined by the known method ofapplying a load required to break the strip.

' Example 1 Cellulose fiber boards were immersed for 4 minutes insolutions kept at 50 C. and containing 25%, 12.5%, 6.25% and 3.125%adipic acid dissolved in isopropanol. The impregnated boards were driedand heated for 20 minutes at C. The resulting boards contained 30%, 16%,12% and 8% of adipic acid, respectively, and showed a reduction inexpansion and contraction in varying humidity of the atmosphere by 94%,64%, 52% and 47%, respectively.

Cellulose fiber boards impregnated with iso propanol solutionscontaining 25%, 12.5%, 6.25% and 3.125% of a solute impregnant composedof equal quantities of adipic acid and. liquid cresolformaldehyde resincapable of advancement contained, after drying and heating, 24%, 11%, 7%and 4.1% of the above dimensional stabilizing impregnant and showed 84%,56%, 42% and 34% of reduction in their contraction and expansion,respectively.

Impregnated and subsequently dried and heated boards containing 29%,16%, 11% and 3.2% of stabilizing ingredients composed of equalquantities of adipic acid and urea-formaldehyde resin possessing thecharacteristic of advancement to an infusible, insoluble stageonlheating, lost 81%, 49%, 24% and 17% of their contraction andexpansion, respectively.

All impregnated boards possessed water-ro sistance, rigidity andstrength superior to those of unimpregnated boards.

Example 2 Cellulose fiber boards impregnated with sebacic acid solutionsof various concentrations in isopropanol contained, after drying andheating, 4.2%, 9%, 16% and 27% of sebacic acid and showed losses of 42%,61%, 91% and 95%, respectively, in their contraction and expansion invarying humidity of the atmosphere.

Impregnated and subsequently dried and heated boards containing 2.2%,3.2%, 12% and 7 21%v of active ingredients composed of equal quantitiesof sebacic acid and liquid cresolformaldehyde resin lost 35%, 47%, 61%and 82% of their contraction and expansion, respectively.

Impregnated and subsequently dried and heated boards containing 3.1%,8.7%, and 27% of active ingredients composed of equal quantities ofsebacic acid and liquid urea-formaldehyde resin lost 27%, 61%, 85% and93% of their contraction and expansion, respectively, in varyinghumidities of atmosphere.

Example 3 Cellulose fiber boards were impregnated with solutionscontaining varying quantities of mixed aromatic polycarboxylic acidsdissolved in a solvent composed of 20 parts of isopropanol and 80 partsof water. In accordance with the information of the supplier, the mixedaromatic acids were prepared by controlled oxidation of bituminous coaland contained three or more carboxyl groups per molecule of benzene.After evaporation of solvents and heating of the board for 15 minutes at125 C. some of the boards lost more than 50% of their contraction andexpansion. A substantial improvement in dimensional stability was alsoobtained when the dimensional stabilizing impregnant was composed ofequal parts of mixed aromatic acid and urea-formaldehyde orcresol-formaldehyde resins. All impregnated and subsequently heatedboards showed improvement in water-resistance and flexural strength.

Example 4 Cellulose fiber boards were impregnated with solutionsmaintained at 50 C. and containing various concentrations of malonicacid alone and its combinations with either cresol-formaldehyde orurea-formaldehyde resin. After evaporation of solvents and heating allof the impregnated boards showed an improvement in dimensionalstability, water-resistance and flexural strength.

Example 5 Impregnating solutions were prepared by dissolving 50 parts ofmaleic anhydride in 150 parts of water kept at 50 C. Cellulose fiberboards were immersed into the solution and kept beneath its surface for4 minutes, at the end of which period the impregnation was complete. Theabove solution was subsequently diluted with an equal weight quantity ofwater and another strip was impregnated with the solution obtained. Thedilution and impregnation was then again repeated twice. The boards thusobtained contained, after drying and heating, various amounts of solidsabsorbed, ranging from 7% to 39%, and lost between 42% and 68%,respectively, of their contraction and expansion in varying humidity ofthe atmosphere.

Cellulose fiber boards were impregnated with solutions of variousconcentrations containing a dimensional stabilizing impregnant composedof equal parts of maleic anhydride and cresolformaldehyde resindissolved in a solvent composed of equal parts of isopropanol and water.The impregnated boards, after their drying and heating at 125 C. forminutes, contained 4.2%, 12% and 14% of active ingredients and lost 14%,36% and 43% of their original property of contracting and expanding invarying humidity of the atmosphere. r I

An improvement in dimensional stabilization was also obtained when theboards were impregnated with a solution prepared by dissolving maleicanhydrides and urea-formaldehyde resin in 20% aqueous isopropanolsolution followed by heating of the impregnated boards at C. for 30minutes. All impregnated boards showed improvement in water-resistanceand fiexural strength.

A still further improvement in dimensional stability of impregnatedboards was obtained when they were subjected to moderate pressure duringtheir heat treatment, on the order of 200 lbs. per square inch.

Example 6 Impregnating solutions containing 3.125%, 6.25% and 12.5% offumaric acid were prepared by dissolving required quantities of the acidin isopropanol. Cellulose fiber boards impregnated with the abovesolutions contained 3.2%, 7.2% and 19.8% of fumaric acid, respectively,and showed losses in their original property of expanding andcontracting in varying humidity of the atmosphere by 27%, 31% and 51%,respectively.

Impregnated and subsequently heated boards containing 2.1%, 4.2% and12.5% of active ingredients composed of equal quantities ofcresolformaldehyde resin and fumaric acid lost, after their drying andheating, 30%, 36%, and 44% of their original property of contracting andexpending due to varying humidity conditions of the atmosphere.

Impregnated and subsequently heated boards containing variouspercentages of fumaric acid and urea-formaldehyde resin showed alsoimprove-d dimensional stability.

All impregnated and heat treated boards possessed improvedwater-resistance and fiexural strength.

. Example 7 Cellulose fiber boards impregnated with isopropanolsolutions of succinic acid of various concentrations and containing,after heating, 5.2%, 9% and 17% succinic acid, lost 34%, 44% and 70%,respectively of their original property of contracting and expandingwith variation of humidity in the surrounding atmosphere.

Cellulose fiber boards possessing improved dimensional stability werealso obtained when they were impregnated with solutions containingsuccinic acid and urea-formaldehyde or cresolformaldehyde resin andsubjected to heat treatment thereafter. The boards thus treated alsopossess improved rigidity, water-resistance and fiexural strength.

Example 8 Impregnating solutions were prepared containing variousconcentrations of a solute composed of phthalic anhydride alone or inconjunction with one of the thermosetting resins, such asurea-formaldehyde or cresol-formaldehyde, and a solvent composed of 1part of isopropanol and 4 parts of water. Cellulose fiber boardsimpregnated with these solutions possessed, after heating, improveddimensional sta bility, rigidity, water-resistance and flexuralstrength.

Example 9' impregnating solutions were prepared by dissolving inisopropanol required quantities of carbic anhydride to contain 3.125%,6.25% and 12.5% of the latter.

Cellulose fiber boards impregnated with above solutions maintained at50' C. and subsequently heat treated, contained 4.2, 7.2% and 10% carbicanhydride and showed a reduction in contraction and expansion by 19%,26% and 35%.

Cellulose fiber boards impregnated with solutions of variousconcentrations containing equal quantities of carbic anhydrides andcresol-formaldehyde resin dissolved in isopropanol contained 3.2%, 6.3%and 10% of above active ingredients and lost, after drying and heatingfor 30 minutes at 125 C., 22%, 28% and 35% of their original property ofcontracting and expanding in varying humidity of the atmosphere.

Where impregnation of the fibers is attempted prior to the preparationof a fiber board, economic and operational restrictions will narrow theselection of the polycarboxylic acids employed under such circumstancesto those which are soluble in water. Comminuted cellulose fibers can beimpregnated, however, with the stabilizing chemicals dissolved inorganic solvents and structural members made therefrom show excellentdimensional stability under extremes of humidity conditions. This isshown in the following example:

Example A An aqueous pulp suspension ofa consistency of 1% was preparedcontaining 18% concentration of adipic acid based on solution, Sheets offiber board were prepared from this pulp, cut to size and the expansiondetermined by increasing the humidity from to 90%. When this expansionwas compared with that of board I.

made from another portion of the same pulp without the presence of thestabilizer, it was found that a 13% content of the acid in the board,based on the Weight of dry fiber, eliminated 45% of the normalexpansion.

The same type of results were obtained when applying a solution of thestabilizin chemicals to the wet end of the paper making machine. Thisoperation gives somewhat greater flexibility in the choice ofstabilizing compound to be employed, as compared with addition to thebeater or head chest, for example, since it is entirely practicable touse organic solvent solutions of the stabilizer, for instance, asolution made of equal parts water and isopropyl alcohol and containing2% concentration of sebacic acid. When applying such a solution to thewet lap in amounts to provide 2.5% of acid in the board on a dry fiberbasis, reductions in the normal expansion of 26% were obtained. At theselower dilutions, good results were obtained but, in many instances,operating technique will dictate the employment of relativelyconcentrated solutions when application is made to the wet lap.

Laminated cellulose structural fiber board may be impregnated with thedimensional stabilizer in any suitable fashion although immersion in thedimensional stabilizer or a solution thereof is recommended. In general,the temperature of the liquid in which the laminated cellulosestructural fiber board is immersed will be at room temperature. Wherea'laminated product of an exceptionally high caliper is to beimpregnated, the temperature of the liquid may be elevated to facilitatepenetration. The laminated board may be soaked in the impregnatingsolution until such time as the desired quantity of dimensionalstabilizer has been absorbed by or combined in some physico-chemi'calmjanner with the cellulose.

It may be found expedient when treating laminated cellulosestructural'fiber boards, or other cellulose elements which are relatively rigid, topack the same in a chamber preferably in an upright position, having theboards spaced slightly apart to facilitate free circulation. It willalso be found expedient to subject the chamber to vacuum whereby gasesand other volatile materials which interfere with free penetration ofthe solution into the board, are removed. Liquid containing thedimensional stabilizer is then admitted'to the evacuated chambercontaining the cellulose material and penetration throughout the body ofthe cellulose elements is facilitated. The impregnated boards are thenremoved from the solution and passed through any conventional'form ofdrier.

Examples of polycarboxylic acids which may be employed. in the presentinvention are oxalic, malonic, succinic, methylmalonic, glutaric,propane-alpha-beta dicarboxyli, ethylinalonic, butans-alpha deltadicarboxylic, adipic, betamethyl a propane alpha beta dicarboxylic,mixed polycarboxylic aromatic acids obtained by the controlled oxidationof bituminous coal, pimelic, diethylmalonic, suberic, azelaic, sebacic,decamethylene-dicarboxylic, tridecandioic, tetradecandioic,octadecandioic, nonadecandioic, fumaric, maleic, glutaconi mesaconic,citraconic, muconic, alpha-carboxyglutaric, beta-carboxypimelic,camphoronic; aconitic, cyclopentendioic, l,l-hexahydrophthalic,camphoric, phthalic, 2- carboxy-phehylace'ti, phenylsuccinic,benzalmalonic, naphthalene dicarboxylic 1,8; diphenyl-carboXyIic-ZB;cyclopropane-tetracarboxylic 1 .l,2,2 naphthalene tetracarboxylic-1,4,53; dimer-abletio, dimer-linoleic, dimer-linolenic and the like andtheir anhydrides, such as phthalic, maleic, succinic and carbic (3,6en'domethylene-tetrahydrophthalic) anhydrides.

Polycarboxylic acids, in general, have the requisite physicalcharacteristics, for employment as stabilizers'in accordance with thepresent invention. One of the important physical requirements of thestabilizer is thatit be'solublein water or volatile water-misciblesolvents such as lower aliphatic mono alcohols or ketones, ormixturesthereof. Also, in general, the greater its solubility-in one ofsuchvolatile solvents, the greater its penetration into the cellulose fibersand the greater its'stabilizing' effect. The latter is true regardlessof whether the stabilizing material is employed in solvent solutionduring. impregnation.

Another important physical characteristic of the stabilizer is that itbe substantially non- .volatile under all temperature conditions likelyto be encountered. That is to say, it should have a boiling pointat-least as highia's C. and prefera'bly substantially higher atatmospheric pressure.

As to the solvents which may be emplo ed for making p an impregnatingsolution, water is the preferred solvent and will ordinarily beemployed-alone if the stabilizing material is soluble therein in allpropertions. If necessary to obtain solution of the stabilizingmaterial, volatile water-miscible organic solvents such as monohydroxylaliphatic alcohols, containing three cal bons orless or aliphaticketonescontaining five carbons or less maybe employed either alone or 1nadmixture with each other or with water. By way of example, methyl,ethyl and propyl alcohols are particularly suitable and dimethyl,diethyl, methyl ethyl, methyl "propyl or ethyl 11 propyl ketones arealso suitable. Such solvents or solvent mixtures should have a boilingoint subtantially below that of the stabilizingmixture,i.e., a boilingpoint not above approximately 105 C. at atmospheric pressure. Suchsolvents may be termed volatile hydrophilic solvents and for purposes ofthis application the term volatile hydrophilic solvent is defined aswater, a watermiscible organic solvent or mixtures thereof having aboiling point not greater than 105 C.

In lieu of the polycarboxylic acid as the dimensional stabilizer, theesters of such acids with low molecular weight alcohols may be employed.It is believed that such esters, either by hydrolysis or alcoholysiswith the cellulose, become effective for the physico-chemicalcombination believed to constitute the mechanism of the stabilizingaction.

To summarize, the stabilizing materials in accordance with the presentinvention are either aromatic or aliphatic polycarboxylic acids ormixtures of such acids. In addition to such chemical requirement, thestabilizing materials should be soluble in all proportions with at leastone of the volatile hydrophilic solvents as defined above and shouldhave a boiling point at least as high as 150 C. Since solubility involatile hydrophilic solvents depends upon several factors such as thenumber of hydrophilic groups, for example, carboxylic groups, thesaturation of the compound and arrangement of carbons as well as thenumber of carbons, it is impossible to more definitely specify thenature of the effective compounds by chemical characteristics. The samesituation exists as to the boiling point of the effective compounds.

The concentration of the impregnating solution can vary from 100%polycarboxylic acid in the case of low-viscosity polycarboxylic acidsdown to a concentration as low as 1%. In general, however, the bestresults are obtained when polycarboxylic acids are employed in asolution having a concentration ranging between approximately 5% and60%. Also, in general, the dimensional stability obtained does notdecrease proportionately with the decrease in concentration of theimpregnating solution. When very dilute solutions are employed, adisproportionately high degree of stabilization is obtained. Thus theconcentration of the polycarboxylic acid in the impregnating solutionmay range from approximately 1% to 100% and the amount of polycarboxylicacid retained in the fibrous material may range from approximately 1% to50%, the preferred range being between approximately 3% and 50%.

The temperature or the impregnating solution during impregnation has aneiiect upon the results obtained. That is to say, the rate ofpenetration of the polycarboxylic acid into the fiber structure, thequantity of deposition of the acid in the fiber structure and theresulting improvement in dimensional stability not only depends upon theconcentration of the polycarboxylic acid in the solution but alsodepends on the temperature of the impregnating solution. The higher thetemperature at which impregnation is carried on, the greater the amountof polycarboxylic acid retained in the fibrous structure for a givenconcentration of the impregnating solution, and the more effective isthe dimensional stabilization. The usual impregnating temperature willrange between and 60 C., although any temperature below that at whichtween 3 and 60 minutes.

rapid vaporization of the impregnating solution takes place may beemployed. Such temperature will not ordinarily be above C.

The polycarboxylic acids deposited in the li brous products exhibitconsiderable resistance to removal by water and solvents, particularlyif the impregnated fibrous products are heated during drying orthereafter to temperatures between approximately 100 and 200 C. Thisindicates the possibility of a reaction between the cellulose and thepolycarboxylic acids to modify the cellulose. Such modification of thecellulose is beneficiall reflected by the increased dimensionalstability and the increased strength and decreased waterabsorptivity ofthe resulting pmduct.

The affinity of the stabilizing materials of the present invention forcellulose fibers, their penetration powers and the reason for theirfixation on and in the cellulose fibers is not fully understood. From aconsideration of their molecular structure, it may be supposed thattheir affinity for cellulose fibers depends upon the presence ofcarboxylic groups in the polycarboxylic acid and the presence ofhydroxyl groups in the cellulose. Their power of penetration is possiblydue to the presence of balanced hydrophilic and hydrophobic groups inthe molecules which is characteristic of surface active materials. Thesetwo properties apparently facilitate the deposition of the stabilizingmaterials in and on the cellulose fibers.

In processing the impregnated cellulose fiber materials, heating to atemperature which produces further reaction of the stabilizing materialboth with itself and with the cellulose is desirable in order to obtainthe maximum effect of the present invention. Such temperatures willusually lie between 100 and 200 C. but in any case, the uppertemperature should not exceed that at which scorching or volatilizationof the. stabilizing material may occur during the period of heating. Thepermissible upper temperature limit varies with the duration of heatingand also with the type and quantity of impregnating substances in thecellulose material. The time of heating also varies with the temperatureand the type and amount of stabilizing material or mixture withthermosetting resin retained in the cellulose fiber sheets but willusual y range be- Such heating is ordinarily required to obtain theprotective effects of the thermosetting resins when such resins areemployed as part of the impregnant but substantial dimensionalstabilization approaching that obtained after a heat treatment usuallyresults from merely drying the impregnated structures at lowtemperatures, regardless of whether thermosetting resins are present,

My invention provides an impregnating solution and method forimpregnating cellulose fiber materials which increases the rate anddepth of penetration of the impregnating solution, improves the fiexuralstrength of the cellulose fiber material and stabilizes it againstcontraction and expansion due to varying atmospheric humidity While atthe same time reducing its ab sorption of water. Being acidic in nature,due to the carboxyl groups therein, the polycarboxylic acids of myinvention are particularly adapted for use in combination withthermosetting resins, increasing the curing rate of the resins,eliminating the use of mineral acid catalysts for the resins and theirdetrimental effects, retarding the gelling of the resins and prolongingtheir useful life, while the resins, on the other hand,

13 improve the fixation and water-resistance of the stabilizingcompositions and reaction products thereof deposited in and on thecellulose fibers. In many instances, the polycarboxylic acids serve aspowerful solvents for the thermosetting resins, thereby reducing theviscosity of the impregnating solutions, and beneficially affecting theimpregnation of the cellulose fiber materials, Through such advantageouscharacteristics, my invention provides a method for impregnating andimproving materials made up of cellulose fibers without the necessity ofresorting to the timeconsuming, laborious and expensive steps ofarranging laminations with crossing fibers, or applying heavy pressureswhich densify, crush and injure the fiber structure.

What is claimed is:

1. The method of stabilizing fiber wall boards consisting essentially offelted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with betweenapproximately 1% and 50% of at least one polycarboxylic acid, saidpolycarboxylic acid being in solution in a volatile hydrophilic solvent,and drying the resultin impregnated boards, said polycarboxylic acidbeing soluble in said volatile hydrophilic solvent and having a boilingpoint at least as high as 150 C.

2. The method of stabilizing fiber wall boards consisting essentially offelted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with a solution in avolatile hydrophilic solvent of at least one polycarboxylic acid anddrying the resulting impregnated boards, said polycarboxylic acid beingsoluble in said volatile hydrophilic solvent and having a boiling pointat least as high as 150 C., and said solution having a concentration ofsaid polycarboxylic acid between approximately 1% and 60%.

3. The method of stabilizing fiber wall boards consisting essentially offelted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with a solution in avolatile hydrophilic solvent of adipic acid and drying the resultingimpregnated boards, said solution having a concentration of said adipicacid between approximately 1% and 60%.

4. The method of stabilizing fiber wall boards consisting essentially offelted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with a solution in avolatile hydrophilic solvent of sebacic acid and drying the resultingimpregnated boards, said solution having a concentration of said sebacicacid between approximately 1% and 60%.

5. The method of stabilizing fiber wall boards consisting essentially offelted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with a polycarboxylicaromatic acid in solution in a volatile hydrophilic solvent, and dryingthe resulting impregnated boards.

6. The method of stabilizing fiber wall boards consisting essentially offelted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with a polycarboxylicaliphatic acid in solution in a volatile hydrophilic solvent, and dryingthe resulting impregnated boards.

7. The method of stabilizing fiber wall boards consisting essentially offelted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with a solution in avolatile hydrophilic solvent of a mixture of a thermosetting resin andat least one polycarboxylic acid, and drying and heating the resultingimpregnated boards to cure said resin, said polycarboxylic acid andthermosetting resin being soluble in said volatile hydrophilic solvent,said polycarboxylic acid having a boiling point at least as high as C.,said solution having a concentration of said mixture betweenapproximately 5 and 60% and said thermosetting resin constitutingbetween approximately 5 and 50% of said mixture.

8. The method of stabilizing fiber wall boards, consisting essentiallyof felted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantial- 1: throughout said boards with a solution in avolatile hydrophilic solvent of a mixture of a thermosetting resinandadipic acid, and drying and heating the resulting impregnated boards tocure said resin, said thermosetting resin being soluble in said volatilehydrophilic solvent, said solution having a concentration of saidmixture between approximately 5 and 60% and said thermosetting resinconstituting between approximately 5 and 50% of said mixture.

9. The method of stabilizing fiber wall boards, consisting essentiallyof felted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with a solution in avolatile hydrophilic solvent of a mixture of a thermosetting resin andsebacic acid, and drying and heating the resulting impregnated boards tocure said resin, said thermosetting resin being soluble in said volatilehydrophilic solvent, said solution having a concentration of saidmixture between approximately 5 and 60% and said thermosetting resinconstituting between approximately 5 and 50% of said mixture.

'10. The method of stabilizing fiber wall boards, consisting essentiallyof felted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with a solution in avolatile hydrophilic solvent of a mixture of a thermosetting resin and apolycarboxylic aromatic acid, and drying and heating the resultingimpregnated boards to cure said resin, said thermosetting resin and saidacid being soluble in said volatile hydrophilic solvent, said solutionhaving a concentration of said mixture between approximately 5 and 60%and said thermosetting resin constituting between approximately 5 and50% of said mixture.

11. As a product of manufacture, a fiber wall board consistingessentially of felted cellulose pulp fibers and containing as animpregnant substantially throughout said boards between approximately 1and 50 of a polycarboxyllc acid,

said polycarboxylic acid being soluble in a volatile hydrophilic solventand having a boiling point at least as high as 150 C.

12. As a product of manufacture, a fiber wall board consistingessentially of felted cellulose pulp fibers and containing as animpregnant substantially throughout said boards between approximately 1%and 50% of a polycarboxylic aliphatic acid, said acid being soluble in avolatile hydrophilic solvent and having a boiling point at least as highas 150 C.

13. As a product of manufacture, a fiber wall board consistingessentially of felted cellulose pulp fibers and containing as animpregnant substantially throughout said boards between approximately 1and 50% of a polycarboxylic aromatic acid, said acid being soluble in avolatile hydrophilic solvent and having a boiling point at least as highas 150 C.

14. As a product of manufacture, a fiber wall board consistingessentially of felted cellulose pulp fibers and containing as animpregnant substantially throughout said boards between approximately 3%and 50% of adipic acid.

15. As a product of manufacture, a fiber wall board consistingessentially of felted cellulose pulp fibers and containing as animpregnant substantially throughout said boards between approximately 3%and 50% of sebacic acid.

16. As a product of manufacture, a fiber wall board consistingessentially of felted cellulose pulp fibers and containing as animpregnant substantially throughout said boards between approximately 1%and 50% of a mixture of a thermosetting resin and at least onepolycarboxylic acid, said thermosetting resin and said polycarboxylicacid being soluble in a volatile hydrophilic solvent and saidpolycarboxylic acid having a boiling point at least as high as 150 C.,the amount of said thermosetting resin being between approximately 5 and50% of said mixture.

17. As a product of manufacture, a fiber wall board consistingessentially of felted cellulose 16 pulp fibers and containing as animpregnant substantially throughout said boards between approximately 1%and of a mixture of a thermosetting resin and adipic acid, saidthermosetting resin being soluble in a volatile hydrophilic solvent, theamount of said thermosetting resin being between approximately 5 and 50%of said mixture.

18. As a product of manufacture, a fiber wall board consistingessentially of felted cellulose pulp fibers and containing as animpregnant substantially throughout said boards between approximately 1%and 50% of a mixture of a thermosetting resin and sebacic acid, saidthermosetting resin being soluble in a volatile hydrophilic solvent, theamount of said thermosetting resin being between approximately 5 and 50%of said mixture.

19. As a product of manufacture, a fiber wall board consistingessentially of felted cellulose pulp fibers and containing as animpregnant substantially throughout said boards between approximately 1%and 50% of a mixture of a thermosetting resin and mixed polycarboxylicaromatic acids, said thermosetting resin being soluble in a volatilehydrophilic solvent, the amount of said thermosetting resin beingbetween approximately 5 and 50% of said mixture.

WALTER P. ERICKS.

REFERENCES CITED The followingreierences are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,857,690 Mellanoff May 10, 19322,050,197 Sebrell Aug. 4, 1936 2,155,731 Mitchell Aug. 25, 19392,185,477 Thompson et a1 Jan. 2, 1940 2,190,331 Mosher Feb. 13, 19402,358,387 Dreyfus et a1 Sept. 19, 1944 2,417,014 Pollard Mar. 4, 1947

